Fire-suppression may refer to a use of agents such as gases, liquids, solids, chemicals and mixtures thereof to extinguish combustion. Fire-suppression systems may use a “total flooding” or a “non-total flooding” method to apply an extinguishing agent in an enclosed volume. The total flooding or the non-total flooding method may achieve a concentration of the extinguishing agent as a volume percent to air of the extinguishing agent sufficient to suppress or extinguish a fire. Use of environmentally friendly fire-suppression agents such as environmentally friendly chemical agents or inert gases are being encouraged as a replacement for Halon in fire-suppression systems. However, some of these gaseous systems may require significantly higher volumetric flow rates and thereby systems with higher volume and weight than existing Halon-type fire-suppression agent delivery systems. In airplane operations, higher volume can reduce revenue generating cargo volume and increase weight, which is undesirable since fuel burn rates increase accordingly.
For cargo fire suppression, a cargo fire-suppression agent Halon 1301 has generally been distributed into a cargo compartment (cargo bay) via dedicated distribution systems. Such dedicated distribution systems are generally optimized for flow rates that discharge Halon 1301 in a high pressure liquid for a High Rate Discharge, and in a gaseous phase for a Low Rate (or metered) Discharge.
In an aircraft application, each cargo compartment may have its own dedicated distribution system comprising tubes routed to nozzles in the cargo bay. The nozzles may be mounted in pans down a centerline of the cargo bay ceiling liner. Fire-suppression systems may be operated automatically by an automatic detection and control mechanism, and/or manually by manual activation of an actuator via a remote switch, a combination thereof, and the like.
To date, aircraft cargo fire-suppression systems generally use Halon 1301 as the fire-suppression agent. Halon is an ozone depleting substance whose production and use has been banned by the Montreal Protocol in the early 1990's. Aviation has had a special use exemption (allowing continued use of Halon) until a suitable replacement is found.
As mentioned above, use of environmentally friendly fire-suppression agents such as environmentally friendly gaseous agents is being encouraged as a replacement for Halon. However, gas discharge volumes for these non-Halon type of suppression systems may require a much higher discharge rate than both liquid discharge volumes and gaseous discharge volumes of Halon 1301. Current Halon-type systems may be limited to low volumetric flow rates of about 150 cubic feet per minute (cfm). Systems that can rely on environmentally friendly gaseous agents or inert gases may require significantly higher volumetric flow rates, on an order of 2000-3000 cfm for an approximate 5000 cubic foot compartment volume, which may be beyond a capability of existing Halon-type fire-suppression agent delivery systems.
The current Halon based systems require a high initial knockdown concentration of fire-suppressant followed by a lower sustained concentration of the fire-suppressant. Current Halon-free cargo fire-suppression systems initially require large volumes of inert gas to be discharged at high mass flow rate followed by low volumes of inert gas to be discharged at low mass flow rate/lower mass flow rate inert gas to be continuously supplied (until landing) to provide an equivalent level of fire fighting performance (compared to Halon 1301). A low mass flow rate inert gas supply may be provided by stored inert gas or an inert gas generator such as a Nitrogen Generation System (NGS); however, the NGS generally cannot in its current design efficiently provide the large volumes of gas at the high mass flow rate needed for high initial knockdown concentration of fire-suppressant.
Limited space is available on aircraft for systems installations. Even at high pressures (about 15 times those currently used for Halon) a comparable non-Halon system may require approximately 10 times a storage space compared to a Halon based system. There is simply not enough free space to install an inert gas based system without sacrificing significant revenue (cargo) volume. Furthermore, current high pressures (e.g., 5,000 psi compared to 360 psi for a Halon system) may require significantly heavier bottles to optimally meet safety and certification requirements (e.g., proof and burst pressure).
This is also a challenge as current Halon based systems are more efficient fire-suppressants than current Halon-free cargo fire-suppression systems from a stored weight and volume perspective. Current Halon based systems generally comprise a dedicated bottle of Halon that stores high pressure Halon and discharges the high pressure Halon at high mass flow rates to meet the initial knockdown requirements. Afterwards a second bottle of Halon is released and slowly metered through a flow restricting device, either an orifice or regulating valve to maintain a lower required sustained concentration. Current inert gas fire-suppression systems are generally not as volume efficient as Halon 1301, which is a reason why no viable Halon-free cargo fire-suppression system has currently been certified and delivered. In current designs, inert gas is stored in high pressure cylinders (e.g., about 5,000 psi working pressure) with shielding to protect from a potential external high pressure loads or puncture, which consumes significant volume and adds significant weight.